The objectives are to further define the factors involved in regulation of epithelial ion transport. Specifically, the site(s) and mechanisms by which putative endogenous regulators of intestinal ion transport alter absorption and secretion of electrolytes by the rabbit descending colon in vitro will be investigated. Measurements of transepithelial ion fluxes, intracellular potentials and ion activities will be conducted. These techniques are designed to further define: (1) the effects of putative endogenous mediators of intestinal ion transport on the transepithelial transport of Na, K and Cl in the colon; (2) the effects of these mediators on intracellular membrane potentials, membrane resistances and intracellular ion activites; (3) the site(s) of action of these mediators; and (4) the role of calmodulin and prostaglandins in stimulus-secretion coupling in the intestine. Results should provide information concerning the effects of endogenous regulators on intestinal ion transport and the sites and mechanisms by which absorptive and secretory stimuli act. Such information is essential to allow scientifically rational therapeutic approaches to the treatment of disorders of salt and water transport by the intestine. Furthermore, since the transport characteristics of a variety of absorptive and secretory epithelial are similar, studies of the absorptive and secretory mechanisms present in this epithelia may provide important information concerning the pathophysiology of intestinal transport processes in particular and the physiology of absorptive and secretory epithelia in general.